It took until the sixth morning of finals, the sixth gold medal and the sixth world record before the "D" word was raised. "So, Michael, with all the talk of amazing performances of Marion Jones [plus a list of several other US athletes banned for doping or linked to drugs scandals] ... what would you say to those who say that your performances are too good to be true?"

Phelps, who is aiming to break Mark Spitz's 1972 record of seven golds at a single Games tomorrow in the 100m butterfly and would soar past the legend on Sunday in the medley relay if the USA comes up trumps, shrugged and replied: "They can say what they want. I am clean. I did 'Project Believe' with USADA (US Anti Doping Agency) where I purposely wanted to do more tests to prove that. People can question all they want but the facts are facts and I have the results to prove it."

"Project Believe" is a voluntary testing programme that also includes sprinter Allyson Felix and decathlete Brian Clay where the participants provide additional blood and urine samples beyond the regular testing regime. Phelps's image features prominently on USADA's website and he is also featured in the organisation's 'My Victory' campaign, which encourages young athletes to take a pledge against doping.

Phelps, 23, now has an unprecedented 12 career Olympic gold medals, three more than previous record holders Paavo Nurmi (FIN, track and field), Larysa Latynina (URS, gymnastics), Spitz (USA, swimming) and Carl Lewis (USA, track and field). The superstar of the Water Cube and a man who has sparked a debate about what constitutes "the greatest", noted that his training regime is constant, tough and specialised. He had got to where he was on a diet of hard work.

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"With a programme like I have, I have to work on speed and endurance and all four strokes. I need the speed to take it out (at the start) and the endurance to carry it through," he said. "You can't do it on talent alone. A lot of hard work, a lot of dedication."